Collagen and proteoglycan are considered as molecular indices to the biological aging process. This consideration is especially attractive in such tissues as tendon, where the functional constituents are little else. The aim of this research is to measure the interrelationship of these molecules directly in tendon without chemical or physical disruption. Changes are correlated with normative aging, and with retarded or accelerated aging in rats as functions of varying nutrition. Luminescent probing is used for direct observation of tendon preparations. Information derives from ultraviolet light activated electron excited states (principally triplet, or as phosphorescence) of tail tendons supercooled to as low as 4.2 degrees Kelvin. The luminescent center in collagen is tyrosyl residues, and in proteoglycan as tryptophanyl residues. The excited states of these aromatic amino acids varies significantly. Thus each can be differentiated simultaneously. A fully compensated spectrophotofluorimeter has been modified to handle tendon fibers in a helium refrigerator. Optical data from the instrument are discerned and denoted by a computer, and changes in quantum functions are recorded in terms of initial population of excited states and their lifetimes.